1. Field of the Invention
The present invention relates to industrial type non-contact detection systems for ascertaining the state of an object in a machine system. More specifically, the present invention relates to a laser detection system for ascertaining the state of rotating objects or objects in some other continuous repetitive motion.
2. Description of the Prior Art
The development of machinery for automated manufacturing has produced many benefits to mankind. More specifically, today, the manufacture of most consumer and industrial electronic instruments and appliances is largely automated. During the manufacture of electronics boards, holes must be precisely drilled in a circuit board for interconnecting circuits on the board and for fastening the boards to a chassis. For this purpose, high speed, computerized drilling machines rapidly position the board, hold it in place, and drill the necessary holes.
However, problems arise when a drill bit breaks during the drilling operation and the broken bit goes undetected. If the broken bit is not detected, the drilling machine will attempt to continue drilling holes with serious consequences. On the one hand, if the drill breakage occurs only a small distance from the tip of the bit, the drilling machine will continue to drill with the damaged drill bit, thus destroying the board or, at best, producing unacceptably poor quality holes and, in either case, this may require the defective circuit board to be discarded. On the other hand, if the drill breakage occurs at a greater distance from the drill tip, the resultant circuit board will lack holes which were to be created in the drilling process subsequent to the bit being damaged. A defective board lacking one or more holes may be recognized during a post-inspection process and the missing holes may be drilled. However, this corrective procedure causes a significant reduction in productivity.
While non-contact systems for detecting broken bits are presently available, conventional non-contact systems are typically highly affected by environmental conditions and are unable to be conveniently located on the drilling system. Presently known non-contact detection systems include those using light emitting diode (LED) or microwave technologies, both of which are highly susceptible to environmental conditions. These non-contact detection systems do not utilize the benefits of the continuous repetitive movement of the object to eliminate error, interference and the effects of harsh changing environmental conditions.
Also, many available detectors are required to be located very close to the object and at an angle normal to the objects motion. Generally, this is a poor location and subjects the detection device to difficult operating conditions. In a drilling machine operation, the traditional light detection systems must be placed in a ballistic path of the debris generated by the drill increasing interferences and device errors.
LED detection systems utilize the principle of light occlusion thus requiring the object being detected to be directly in the path of the light. On drilling systems, this necessitates the full retraction of the drill bit into a pressure foot assembly. This full retraction after each hole is drilled greatly decreases the efficiency of the drilling machine. Also, such a system must detect small fractional signal changes, particularly for drills with smaller diameters. However, the resulting LED, or occlusion, system has a relatively low signal-to-noise ratio making such signal ranges difficult to detect.
Non-contact microwave detection systems operate on the principle that the presence of a continuous target, such as a fine drill, in or near the end of the waveguide at a predetermined location produces a change in the standing wave ratio (SWR) of the transmitted signal. This change in the SWR is capable of being detected by a receiver to provide a signal that is indicative of the target condition. These microwave systems do not operate using the Doppler principle of measured frequency change; rather, only the DC component of the signal is utilized by such detection systems. Hence, the frequency components of the output signal are not used.
The creation of a resonant microwave cavity and the detection of objects entering the cavity by a change in the SWR is affected by nearly any foreign object entering the resonant cavity. Thus, such a system is prone to interference from airborne debris. The microwave detection systems are also subject to interference from the copper shavings produced during the drilling of copper layers in printed circuit boards, known as "drill-wrap", and system to system variations.
In addition, all elements in or surrounding the microwave cavity must remain constant. The components making up the cavity cannot be moved without making changes or adjustment to the microwave detection device. The microwave detection system is not easily adaptable from system to system and adjustments or variations may be needed depending on the objects in and around the cavity created by each host system.
The microwave detection systems produce small signal changes from the object-present state to the object-not-present state which are analyzed by the system's comparator. Using a small signal change greatly increases the possibility of error by the microwave system.
In addition to the aforesaid disadvantages of the presently known drill bit detections systems, these systems do not detect when a drill bit is clogged. Because of the environmental affects on LED detection systems, and the resulting low signal-to-noise ratio, it is difficult to obtain a signal that is measurably indicative of the varying reflectivity of a spinning drill bit. It will also be appreciated that the present microwave systems utilize only the DC component of the reflected signal and, therefore, do not take into account the frequency of the reflected signal.
Many of the disadvantages of the LED and microwave systems have been overcome by the advent of a laser detection system which detects fluctuating light reflected from a drill bit. Such a laser detection system is presently included in the Concept 1 & 4, MVI and MVII drilling machines manufactured by Excellon Automation of Torrance, Calif. Nevertheless, while the present laser detection system is advantageous for detecting the presence or absence of a drill bit, it does not detect whether a drill bit is clogged.
A drill bit may be become clogged, for example, when "green" printed circuit boards are being drilled. Circuit boards are typically formed from an epoxy resin material which must be cured. Therefore, a green, or incompletely cured, board contains tacky material that may stick to the drill bit during drilling. A drill bit may also become clogged when the feed rate of the drill bit is too low. That is, if the axial motion of the bit through the work is too slow, the drill bit will heat up, heating surrounding board material by conduction, and again providing a means for tacky material to be deposited in the flutes of the drill bit. A clogged bit has a reduced reflective surface area that causes the present laser detection system to sometimes indicate that the drill bit is broken. Regardless of how material is deposited in the flutes, it is often important to distinguish between a clogged drill bit and a broken drill bit.
In general, when a drill bit breaks, the drilling of a circuit board must stop until the drill bit is replaced. On the other hand, when a drill bit is clogged, the drilling process need not necessarily stop. Oftentimes, the drill bit will clog but it is unnecessary to clean the drill bit until many boards have been drilled. Therefore, if the bit detection system cannot distinguish between whether a bit is broken or just clogged, drilling time is lost due to unnecessary delays in checking clogged bits. It is also vital to provide the user of the drilling machine with the option of either stopping the machine or continuing the drilling.
The needs for broken and clogged drill bit detection have been addressed by the invention disclosed in the copending U.S. application Ser. No. 07/791,416.
A further disadvantage of LED and microwave systems, as well as the aforementioned laser detection systems, such as those included in Excellon Automation's Concept 1 & 4, MVI and MVII drilling machines, is that they are not able to determine whether the drill bit currently being used has a sufficient flute length for a particular drilling operation. In some drilling applications, where the holes drilled must be of an exact depth, such as drilling circuit boards, the drill bits used must have flutes extending along the drill bit for a specified length to be able to drill these holes to the exact depth required.
Drill bits used for drilling holes in circuit boards typically include a tip, with a cutting surface, a fluted section, and a shank section. The tip and cutting surface is used to actually bore the hole into the circuit board. The tip and cutting surface is then followed by a fluted section, having a diameter roughly equal to or less than the diameter of the cutting section, which is used to remove material bored by the cutting surface from the hole. At the end of the fluted section opposite the tip, the drill bit includes a shank section which typically has a larger diameter than the fluted section to permit the shank to be retained by the collet of the drilling machine during drilling operations.
If the drill bit is used to drill a hole which is deeper than the length of the fluted section of the drill bit, the larger diameter shank section will then come into contact with the upper surface of the material being drilled when the drill bit has bored a hole along the full length of its fluted section. This may result in damage to the material being drilled. For example, if the larger diameter shank section of a drill bit comes into contact with the upper surface of a circuit board during drilling, the circuit board is likely to be ruined as the shank section will typically bore a hole in the upper surface of the circuit board with a diameter which is substantially larger than the diameter that was intended. An additional problem that often results when the shank section of a drill bit, particularly a small drill bit, e.g., having a diameter of 0.0020 inches, makes contact with the upper surface of a current board is that the drill bits will often break as a result of the additional stresses placed on them.
The problems stemming from a drill bit having insufficient flute length usually result from either the operator of a drilling machine placing the wrong drill bit into the collet of the drilling machine or, in automated drilling applications, the drilling machine selecting the wrong sized drill bit. While drill bits are often marked or otherwise identified so that an operator or a machine can select the appropriate drill bit, these markings are often not reflective of the true flute length of the drill bit. Specifically, when the cutting surfaces of drill bits are sharpened, a small portion of the fluted section of the drill bit is often ground away. Repeated sharpening of a drill bit can result in an appreciable decrease in the flute length of a drill bit from the marked value of the flute length. Consequently, even though the operator or the automated drilling machine is selecting an appropriately marked drill bit, the flute length of the drill bit may still be insufficient for a specific drilling operation.
Heretofore, the flute length of a drill bit was typically verified by off-line measurements of the drill bit. However, many drill bits are very small which makes measurement of the lengths of the flutes by conventional means very difficult. Further, in automated drilling applications, off-line measurement of drill bits intended to be used in an automated drilling sequence would result in a substantial reduction in the efficiency of the automated drilling machine.
Consequently a need exists for a drill bit detection system which will quickly measure a drill bit to ensure that it has a sufficient flute length to drill holes of the intended depth. Further, a need exists for a drill bit detection system which can measure the flute length of a drill bit on-line as part of the automated drilling sequence of an automated drilling machine.